In rolling bearings, seals have often been disposed between the outer ring and the inner ring for the purposes of preventing the grease present around the rolling elements and the dust generated during use from leaking out and of preventing external suspended dust particles from coming into the part where the rolling elements are disposed. An example of such sealed rolling bearings is shown in FIG. 1.
The rolling bearing shown in FIG. 1 is a sealed bearing having a seal on each side. This seal 1 is constituted of a ring-form core metal 2 having a hook part along the outer edge and, disposed on the outer side of the core metal 2, an elastic body 3 formed by molding and vulcanizing a synthetic rubber integrally with the core metal. When viewed from the standpoint of functions, this seal is divided into: a ring-form main part 11 composed of the core metal excluding its hook part and that part of the elastic body which is located on the outer side of that part of the core metal; a caulking part 12 which is composed of the hook part of the core metal and that part of the elastic body which is located outside the hook part, the caulking part 12 being fitted into a fixing groove 41 formed in the inner circumferential surface of the outer ring 4; and a lip 13 consisting of that part of the elastic body which is located on the inner circumferential side of the core metal, the lip 13 being brought into sliding contact with a receiving groove 51 formed in the outer circumferential surface of the inner ring 5.
The caulking part 12 is elastically deformed and forced into the fixing groove 41 in the inner circumferential surface of the outer ring while keeping the lip 13 in contact with the receiving groove 51 in the outer circumferential surface of the inner ring. Thus, the seal 1 is fitted into the space between the outer ring 4 and inner ring 5 of the rolling bearing.
On the other hand, as shown in FIG. 3, an automotive hub unit 6 also has a seal 10 attached to the outer circumference thereof. This seal 10 also is composed of a core metal 20 and an elastic body 30 constituted of a molded rubber material. This hub unit 6 comprises a hub 61 into which a shaft is to be disposed and a flange 62 integrally formed with the hub 61. The seal 10 has two lips 31 and 32 to be in sliding contact with the hub 61 and one lip 33 to be in sliding contact with the flange 62.
There also is a hub unit bearing which comprises a hub 61, a flange 62, and interposed therebetween a hub shaft part for shaft fitting therein, and in which the outer circumferential surface of the hub shaft part has an inner raceway for a rolling bearing. A seal has been attached also to this type of hub unit bearing.
An example of still another type of seal is shown in FIG. 5. This seal 100 also is composed of a core metal 200 and an elastic body 300 constituted of a molded rubber material. This seal 100 serves to seal the space between a member 50 corresponding to an inner ring and having a raceway surface for rolling elements 70 and a member 40 corresponding to an outer ring. The member 50 corresponding to an inner ring comprises a hub part 50a and a flange part 50b. This seal 100 has a radial seal lip 301, which is brought into sliding contact with the outer circumferential surface of the hub part 50a, and two side seal lips 302 and 303, which are brought into sliding contact with the flange surface of the flange part 50b. 
The materials of those seals are generally as follows. The core metals are plates of a steel such as SPCC or SECC. The elastic bodies constituting the lips and other parts are synthetic rubbers such as nitrile rubbers, acrylic rubbers, silicone rubbers, and fluororubbers.
In the case where such rolling bearings and automotive hub units are used in a severe environment in which water and dust are present in large amounts, the following trouble may arise. During continuous use, the lip of the seal comes to have reduced elasticity or breaks, and the force of sliding contact of the lip with the seal contact surface (that surface of the mating member which is in contact with the lip of the seal) decreases accordingly to form a minute gap between the lip and the seal contact surface. Water and dust particles then come into the bearing through the gap. As a result, the grease deteriorates and this may lead to a shortened life of the bearing.